The only thing I had heard about the Pope’s new book on the infancy of Jesus was that the ox and ass in Nativity scenes had been declared a myth. This turns out not to be so.

Jesus of Nazareth: the Infancy Narratives was published in America last week and comes out here on Tuesday (Burns & Oates, £12.99). I’d been looking forward to this completion of Benedict XVI’s three volumes on the life of Jesus. I wasn’t disappointed.

He writes as Joseph Ratzinger, as these books are not part of official papal teaching. But it is interesting to look into the Pope’s mind, and in any case he is a leading theologian of the past century.

The new, quite short, book is a sort of church porch or narthex to the other two, and, as at St Mark’s in Venice, it is a place where you can get up close to the narrative mosaic.

“No representation of the crib is complete without the ox and the ass,” the author writes (as pictured here in a painting by Paolo di Giovanni from about 1400). He is well aware that they are not mentioned in the Gospels of St Matthew and St Luke, the evangelists who give accounts of the Nativity. But “prayerful reflection, reading Old and New Testaments in the light of one another,” filled the gap at an early stage of Christianity. After all, the Gospels do say that Jesus was laid in a manger, and mangers suggest animals.

At the beginning of the prophet Isaiah, which contains so many other references to the expected Messiah, comes a verse: “The ox knows its owner, and the ass its master’s crib.”

The Pope here introduces “an idea that at first seems almost shocking, but on closer examination contains a profound truth”. It comes from a meditation on the manger by St Augustine.

Augustine chips away, in his inquisitive manner, at the idea of our eating the bread of angels, as Psalm 78 promises. Angels, being spirits in heaven, he suggests, have the Word of God as their food, and live happily for ever. We, though, are the sheep of God’s flock. How are we to eat the bread of angels? Look at the manger, Augustine says. God who is a spirit in heaven has taken flesh on earth, and now lies in the manger as our food.

Augustine is not advocating ritual cannibalism. But he saw that the arrival of God-made-flesh referred forward (as well as back to the prophets) to the chief means by which believers can share in the divine life of Jesus: Holy Communion. It was no accident that Jesus was born in Bethlehem, which means “House of Bread”.

It is worth noting that the Pope’s new book accepts the historicity of Jesus’s birth in Bethlehem – or rather its geographical truth. He rejects a hypothesis of some modern exegetes that the evangelists “refashioned history theologically” to make Bethlehem the birthplace of Jesus when in fact it was Nazareth. “I do not see how a basis for this theory can be gleaned from the actual sources,” he remarks mildly. Later he refers to the kind of “theology that exhausts itself in academic disputes”. He prefers, in place of methodic scepticism, the “hermeneutic of faith”, as he explains elsewhere, in which Scripture is handed on by the Church, the people of God, and understood by it.

As far as the good old ox and ass go, some early writers applied to them the words of the prophet Habakkuk (in the Greek version of the Bible used before the time of Christ by Jews in the diaspora): “In the midst of two living creatures you will be recognised.” But if Jesus lies between two living creatures, then Habakkuk was also making an obvious reference to the “two cherubim on the mercy-seat of the Ark of the Covenant (Exodus 25:18), who both reveal and conceal the mysterious presence of God”.

So the manger represents a table for the bread that came down from heaven and also the Ark of the Covenant, upon which the glory of the Lord has descended. Quite enough for any Christmas card.